1. Field of the Invention
The invention is directed to light-emitting-diodes (LEDs) and a process for fabricating these devices.
2. Art Background
Light-emitting diodes (LEDs) are a known means for generating an optical signal. An LED operates by applying a voltage to the LED which causes electrons and holes to inject into light-emissive material. The electrons and holes recombine in the light-emissive material and this radiative recombination results in the emission of light.
The amount of voltage required to bring about the desired radiative recombination depends in part upon the efficiency of the radiative recombination. The efficiency of the radiative recombination depends upon how closely the energy levels of the electrons and the holes match the energy levels of the conduction and valence bands, respectively, of the light emissive material in the LED. The closer the match, the less voltage that will be needed to achieve a certain level of light emission from a particular light-emitting material.
The amount of voltage required for an LED to emit light is termed "turn-on" voltage. The turn-on voltage of an LED is a factor which determines whether an LED is suited for a particular application. Dresner, J., et al., "Anthracene Electroluminescent Cells with Tunnel Injection Cathodes", Proceedings of the IEEE Letters, pp. 1868-1869 (Nov. 1970) describes an electroluminescent cell in which anthracene is deposited either on a n.sup.+ -Si wafer covered with 20-40 .ANG. of SiO.sub.2 or aluminum evaporated on a glass substrate. The turn-on voltages for such an electroluminescent cell is about 60 to 400 volts, which is quite high. If such LED's are to be used in current devices, lower turn-on voltages are required.